Vena cava filters are used to capture potentially fatal pulmonary emboli at an anatomical location where they may pose less risk of pulmonary emboli for the patient. Since the vast majority of pulmonary emboli originate from the lower body, filters are mainly placed in the inferior vena cava.
Vena cava filters have been in use since the 1960s in a variety of configurations. Early filters required open surgical placement (Mobin-Uddin Filter; Kimray-Greenfield filter). Since the late 1970s, improvements in delivery were made and numerous filters were developed for minimally invasive percutaneous placement. These filters included the Greenfield filter, the Gianturco Bird's Nest Filter, the Vena Tech LGM filter, the Simon Nitinol Filter and others. More recently, filters have been developed and marketed with the capability of retrieval after relatively long terms of implantation, which include the Bard Recovery Filter, the Cordis Optease Filter and the Cook Tulip Filter.
Although addressing some desirable characteristics of a filter, the majority of the IVC filters presently on the market do not satisfy other desirable characteristics of an ideal filter. The ideal device should capture blood clots while ensuring continued blood flow through the vessel. Blood flow disruption and turbulence often leads to thrombus formation and buildup at and around the filter. Studies have demonstrated that a conical filter configuration provides the optimal filtering efficiency. Filtering efficiency, for the purposes of this invention can be defined as the capability of the device to capture and retain clots of a pre-determined size, the ability to maintain blood flow through the filter in the presence of captured clots, and the capability of dissolving or lysing the clots caught in the filter. Conical designs force clots toward the center of the filter, allowing blood flow passage around the clot. Continued blood flow through the filter when a clot load is present ensures that captured clots are exposed to the lysing action of the blood flow.
Although conical filter configurations currently available on the market provide optimal filtering capabilities, these designs are prone to tilting and misalignment. When not in proper alignment, filtering ability is compromised. The central conical portion of the filter may tilt to the extent that it becomes embedded in the vessel wall. With retrieval designs, the retrieval hook is typically located at the central apex of the cone. If the tilting results in the retrieval hook coming in contact with the vessel wall, retrieval efforts become difficult or may even prevent removal. Laminar blood flow is disturbed, effective lysing of capture clots decreases, and thrombus build-up occurs.
To address the misalignment problem, filtering cones have been designed with alignment mechanisms to prevent tilting. For example, stent-like cage constructions have been designed to prevent the conical filter from becoming mis-aligned. The stent-like cage rests up against the vessel wall providing alignment to the filtering conical portion of the filter. This design, while optimizing centering of the filter, cannot be easily retrieved because of the difficulty in snaring and collapsing the cage. An example of this type of filter design is the Vena Tech LP filter which has a conical filtering segment adjoined to a zigzag stent base configuration for centering the cone within the vessel. Although this type of design combines the optimal filtering characteristics of a conical configuration with a non-tilting base, the device is not retrievable. The struts of the non-tilting base become incorporated into the vessel wall and cannot be easily disengaged and removed using standard snare removal techniques. The location of the stabilizing struts prevents the ability to withdraw the device into a sheath for removal.
It is possible to build a simple centering cage base/cone filter design that is retrievable by attaching the base to the filter segment in series. This design, while retrievable, is not practical due to the increased length of the device. The desired length of a typical IVC filter is between 3 and 5 centimeters. Longer lengths are undesirable because of the limited implantation space of the vena cava. For example, in some cases it is necessary to deploy a second filter due to malfunction of the initially placed filter. Shortening the filter segment may make the overall device length acceptable, but may result in sub-optimal filter strut angles. Alternatively, shortening the centering cage segment may compromise the alignment function of the device.
IVC filters should be capable of remaining in the vessel for long periods of time, and in some cases, indefinitely. The filter should be designed so as not to migrate from its originally deployed position while still allowing for retrieval of the filter. Thus the vessel wall engagement mechanism should be designed so as to maintain position even under a heavy clot load and yet allow easy and a traumatic disengagement from the vessel during retrieval. Longitudinal movement of the filter has traditionally been prevented by configuring filter ends with hooks that embed in the vessel wall.
Because of concerns with permanent implantation of filters, including possible migration and structural integrity over long time periods, there is an emerging trend for filters that can optionally be retrieved after a specified period of time. The optimal retrievable filter should have wall-engaging mechanism that is sufficient to ensure that the device does not migrate in either direction while implanted. The wall-engaging mechanism should also be designed to allow percutaneous removal of the device without significant trauma or damage to the vena cava wall even after neointima overgrowth has occurred. These two disparate clinical requirements, long-term fixation and a traumatic removal, are difficult to achieve in a single filter design. Some prior art filter designs have utilized aggressive anchoring mechanisms to ensure fixation, but these designs are difficult to remove. Conversely, designs that limit wall contact are easier and less traumatic to disengage from the vessel, but may be more prone to migration.
As with all long-term or permanent implant device, the optimal device design will maintain structural integrity of the device for the duration of implantation. Although rare, filter fractures have potentially fatal complications including filter migration into the right atrium and pulmonary embolism caused by compromised filtering efficiency. The ideal filter device should have minimal connection or attachment points which are more susceptible to fatigue over extended periods of time. In addition to long term performance characteristics, it is desirable to provide an IVC filter that is simple and inexpensive to manufacture without requiring complicated assembly processes that might compromise the long-term integrity of the device or increase the overall cost of the device.
Another desirable characteristic of the ideal filter is a small deployment and retrieval system. A design that minimizes the delivery device diameter will result in a smaller insertion site and reduced risks of bleeding, site thrombus and other complications of percutaneous punctures. The ideal vena cava filter should not only be easy to deploy using minimally invasive percutaneous techniques, but also be repositionable during initial deployment. Many filters are designed for ease of deployment but do not allow for repositioning during delivery.